Light makes some materials conductive in a previously unforeseen way. In silicon solar cells, electrons flow when the sun shines. However, scientists at the Stuttgart-based Max Planck Institute for Solid State Research have now come up with a surprise: in a special perovskite, another material used for solar cells, light not only releases electrons, but also electrically charged atoms, known as ions. Moreover, this novel photoeffect is extremely large. Ion conductivity increased by a factor of one hundred. For solar cells made from the material investigated here, the high light-induced ion conductivity is rather damaging; the consequences, however, can now be specifically counteracted. From the point of view of the researchers in Stuttgart, the effect is ground-breaking in itself, as it makes novel, light-controlled electrochemical applications conceivable, such as batteries directly charged by light.